Liquid crystalline oriented films are required to have good transparency and superior electrical and mechanical properties, and polyimide films having such properties find extensive use as the liquid crystalline oriented films.
Since an aromatic polyimide is usually insoluble and infusible, a polyimide film is produced by coating a solution of polyamide acid, its precursor, in an organic polar solvent (N-methyl-2-pyrrolidone) on a substrate, and heating the coating at a high temperature to dehydrocyclize it to a polyimide and cure it. Although the polyimide film so formed has transparency, it is colored brown because of its severe heat history up to its formation. This coloration darkens the visual field to give a poor contrast and impair the function of a display element in which the polyimide film is used. Hence, such polyimide films do not meet the requirements of liquid crystalline display elements of high quality. Various polyimides have therefore been suggested in an attempt to remove the defect of coloration. Of such polyimides, a particularly good aromatic polyimide capable of forming a polyimide oriented film having a small degree of coloration and high transparency is disclosed, for example, in Japanese Patent Application (OPI) No. 91430/83 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"). This polymer has a recurring unit represented by the following formula (IX) ##STR1## wherein R.sub.1, R.sub.2, R.sub.3 and R.sub.4 which may be the same or different each represents hydrogen, a lower alkyl group, a lower alkoxy group, chlorine or bromine, R.sub.5 and R.sub.6 which may be the same or different each represents hydrogen, a methyl group, an ethyl group, a trifluoromethyl group or a trichloromethyl group, and Ar represents a residue of an aromatic tetracarboxylic acid.
Although this polyimide oriented film has excellent transparency, it is colored yellow and has not proved to be entirely satisfactory for use as a liquid crystalline oriented film.